Apply clang-format universally.

* It is tough to contribute to a project that doesn't use a formatter,
* It is extra hard to contribute to a project which has configured the formatter, but ignores it for some files
* Code formatting makes it harder to hide obscure / weird bugs by accident or on purpose,

Let's rip the bandaid off?

Note that PRs currently in flight should be able to be merged relatively easily by applying `clang-format` to their tip prior to merge.

src/libutil-tests/spawn.cc

@@ -33,4 +33,4 @@ TEST(SpawnTest, windowsEscape)
     ASSERT_EQ(space, R"("hello world")");
 }
 #endif
-}
+} // namespace nix